1. Field of the Invention
This invention relates to an image fixing device for use in a recording apparatus such as a copying apparatus of the electrophotographic type, and more particularly to an image fixing device for fixing an unfixed toner on a recording medium by a plurality of heat generating members.
2. Related Background Art
In a recording apparatus according to the prior art, a heat generating member provided in a fixing roller as a fixing member has been great in heat capacity. Therefore, when heat fixing is to be done after a toner image is transferred to transfer paper as a transfer medium, much time is required until the temperature of the heat generating member rises to a desired temperature necessary to melt a toner. Thus, the heat generating member is made to continue it heat generating state from the start till the end of an image forming process. However, the time when the heat generation of the heat generating member becomes necessary is the time when the transfer paper passes through the fixing nip portion and the unfixed toner adhering to the transfer paper is melted and fixed. During the other times, the heat generating member simply radiates heat energy, and this has given rise to the problem that it is not economical and moreover the heat adversely affects the devices around the fixing device.
For this reason, in recent years, the demand for energy saving has also been heightening in recording apparatuses. Particularly in recording apparatuses of the heat fixing type, the electric power consumed by the fixing device is great and in a halogen heater or the like, the rush power during electrical energization is also great and greatly affects the other devices. Therefore, in the latest recording apparatuses, there has been put into practical use a fixing device small in heat capacity and short in wait up time in which the electric power consumed is made as small as possible. On the other hand, the need for recording apparatuses has been widening and the sizes of the paper handled in recording apparatuses ranges widely from A5 to A3.
Therefore, to make the recording apparatus cope with various sizes of paper while reducing the electric power consumed by the fixing device, heat generating members matching the sizes of the paper become necessary and a fixing device having a plurality of heat generating members has been devised. However, even such a fixing device of the energy saving type which has a plurality of heat generating members in accordance with the sizes of paper and can secure a sufficient fixing property under ordinary environment suffers from the problem that the fixing performance is remarkably reduced when the ambient environmental temperature is low and cold paper is to be fixed or when a source voltage applied to the heat generating members is low.